Despite having only been published in March, Fifty Shades of Grey, which has been dubbed “Mummy porn” due to its steamy plot lines, has sold selling more than 10 million copies.

Amazon executives said on Tuesday that the book’s “Kindle edition” was currently outselling the print book at a rate of more than two-to-one.

Fifty Shades of Grey and the next two books in the trilogy, Fifty Shades Darker and Fifty Shades Freed, have combined to make the mother-of-two the bestselling author of the year throughout the whole Amazon site.

Retail analysts said that sales of erotic novels for women were booming because electronic devices such as Kindles allow discreet downloading and reading.

The mother-of-two, a former self-published "Kindle Direct Publishing (KDP)" author, has said that her novels' success are due to offering women a "holiday from their husbands".